The invention developed in the course of operating a steam supply network in urban heating. The distribution takes place at a maximum temperature of about 235.degree. C., the effective pressure being between 21 and 5 bar, according to the locations to which steam is delivered. When the amount of steam supplied is small, or when the pressure rises rapidly in the pipelines, the steam reaches saturation and condensate appears, which it is necessary to discharge in order to keep the steam dry.
Reservoirs known as drain bottles, having their bottom connected to seperators, are disposed at low points of the pipelines, or at intermediate points on long pipelines of uniform slope.
These separators are generally of the so-called "closed float" type; and they comprise, in a chamber which communicates with the drain bottle, a valve leading to an exhaust space, usually the condensate return pipe. The valve, which is biased closed by the pressure obtaining in the chamber, is caused to open by a float acting through a lever. When the condensate in the chamber reaches a certain level, the float opens the valve, and the condensate is expelled into the exhaust space, until, as the level drops, the valve closes.
The device is usually completed by a thermostatic separator, for evacuating the non-condensing gases. As long as the chamber contains condensate and steam only, it is substantially at the temperature of the steam, since any cooling is compensated by condensation of steam. If non-condensables are mixed with the steam, the equilibrium temperature of the chamber is defined by the temperature of condensation of the steam at its partial pressure in the chamber. The thermostatic separator is at a lower temperature, and it opens until the non-condensables have been discharged.
To return to the condensate discharging valve, it should be noted that it works under very severe conditions. It is generally immersed in condensate at a temperature above 200.degree. C., the condensate consisting of pure but highly corrosive water, which may even be contaminated with ammonia, and it discharges an emulsion of water and steam at high velocity, which is thus very erosive, because the throttling of condensate in the valve port causes partial re-evaporation of the condensate. Moreover, when the amount of steam delivered is large, and the pressure in the network drops, the condensate in the chamber re-evaporates. and the closed valve is exposed to dry live steam.
Closed float separating valves generally consist of a ball connected to one arm of a lever whose other arm carries the float, and of a seating with a circular port on which the ball comes to rest. A certain amount of play between the ball and the lever arm which carries it permits correct location of the ball on the port.
It is very important that these valves should provide a first-class seal in the closed position. As a matter of fact, one metric ton of steam costs 60-100 French francs, by reason both of the treatment necessary for steam generator feedwater and of the enthalpy of the steam (of the order of 2.79 gigajoules per metric ton). However, the best separators currently available on the market manifest losses of the order of 3 kg steam per hour. This corresponds to about 7 cc per second at 10 bar, or again 0.8 cc per second of condensate. This may appear a modest amount. However, it corresponds, over a year, to 26 metric tons of steam i.e., to a loss of about 2000 French francs per annum. This amount of money is of the same order to magnitude as the price of the separator itself.
It is very difficult to make a precise analysis of the causes of loss of sealing capacity in separating valves which work at temperatures of the order of 200.degree. C. and pressures up to about 20 bar. This is because of difficulties of in-service observation, and of the small dimensions of these valves. On this latter point, it may be remarked that, in order to limit the overall dimensions of the separators to reasonable values, it has been found necessary to use floats of several hundred cc, at the end of lever arms of about 20 cm length. In order that the float shall be capable for manoeuvering the valve member when it is forced against the port by the pressure which obtains in the separating chamber, the diameter of the port orifice should be less than 5 mm and the stroke of the valve into its open position of the order of a millimeter.
A little thought will show that the causes of the relatively mediocre sealing capacity of conventional separating valves are the impacts of the valve member on the seating during closure, by reason of the condensate discharge speed during separation, and inaccurate self-centering of the valve ball on the periphery of the port. Especially if the ball is not exactly centred when presented to the port at closure, the result can be a wear distortion of a region of the periphery of the port.